Linear electric actuators, i.e. electric actuators with push-pull outputs, are becoming employed in fluid extraction installations as a replacement for the traditional hydraulic linear actuators, typically employed to operate valves. One of the features of such an actuator, particularly for a subsea installation, is that the device operated by the actuator should return to a required position in the event of a failure, such as a loss of electrical control or a mechanical failure. For example, if the actuator operates a valve, then the valve must revert to a closed position, or, more rarely, to an open position, in the event of a failure. There are many actuators available on the market most of which employ an electric motor which drives, via a gearbox, a rotary to linear mechanism such as a screw drive and a small percentage of them have a fail-safe mechanism built in. Those that are available as fail-safe often employ an integral mechanism that ‘rewinds’ the actuator back to its original position in the event of a failure of electric power. The actuator motor winds or compresses a spring when it is powered, so that on power failure the spring returns the actuator to its original position. Typically, the motor drives the linear mechanism to an electrically powered mechanical latch to fully operate a valve, and on failure of the power supply to the latch, the spring returns the linear mechanism to its original position.
A hydraulic actuator normally comprises a simple piston and cylinder and has a fail-safe mechanism provided by the compression of a coil spring so that failure of the hydraulic power source results in the actuator reverting to its initial position by virtue of the potential energy in the spring returning the piston to its original position. Such a mechanism is very simple and reliable and is thus attractive to the fluid extraction contractor, which is one reason why hydraulic actuators have been popular.
The disadvantage of an electric actuator as described above is that the fail-safe mechanism is not simple and has to reverse drive the actuator through its relatively complicated mechanism, which includes the motor, gearbox and rotary to linear mechanism. Furthermore, any failure of the relatively complicated drive mechanism involving seizing or jamming will also result in failure of the fail-safe feature. It is an additional problem that the provision of a fail-safe mechanism may prevent the actuator from being driven in both directions, ie extending and retracting. This is an important feature with several benefits, e.g. there are two methods of retracting the actuator as opposed to the fail-safe only, driving in reverse may allow “freeing-up” of sticky valves and driving in reverse could also give extra capability for wire-cutting operations.
As prior art in the field of linear actuators, there may be mentioned: EP-A-1,024,422; U.S. Pat. Nos. 5,195,721; 5,497,672; WO 01/14775; GB-A-2,266,943; U.S. Pat. Nos. 5,983,743; 5,984,260; 6,041,857; 6,253,843; GB-A-2,216,625; GB-A-2,240,376; U.S. Pat. Nos. 4,920,811; 5,070,944; 6,257,549; GB-A-2,346,429; U.S. Pat. No. 6,152,167; WO 01/86370; WO 01/86371; U.S. Pat. No. 6,176,318; GB-A-2,116,790; GB-A-2,119,172; GB-A-2,120,349; GB-A-2,122,034; GB-A-2,196,414; GB-A-2,255,866; GB-A-2,283,061; GB-A-2,291,949; U.S. Pat. Nos. 5,865,272; and 4,584,902.